Water droplet particles appear dark and muddy (2.8)
I assume this is under Cycles.

Double-check that the Base Color is full white.  By default, a "fresh" Principled Shader uses 80% linear white (i.e., all RGB channels=0.8, not 1.0).  In addition, you'll want to change the IOR to 1.33, so the drops are water, not glass.

Beyond that, you can try replacing the Principled BSDF with a Refraction BSDF (also with 100% white color and IOR=1.33).  This looks brighter in some lighting conditions and darker in others -- overall, I think it looks brighter for drops viewed head-on.

If that's still not bright enough, you may want to try turning the alpha down a bit on the Principled BSDF or consider a Mix Shader to mix a Refraction (or Principled) BSDF and a Transparent BSDF, to let a little extra light through.

Here's a denoised render of four droplets, from left to right -- Principled BSDF with default 80% white and IOR=1.5, Principled BSDF with 100% white and IOR=1.33, Refraction BSDF with IOR=1.33, and mix 70%/30% (i.e., factor=0.3) of Refraction BSDF with Transpartent BSDF.